Conventionally, there are known induction lamp lighting devices for lighting an induction lamp that includes a bulb made of a transparent material, e.g., glass, and a discharge gas enclosed within the bulb (see, e.g., Japanese Patent Laid-open Publication Nos. 2005-158464 and 2005-158459). This kind of lighting device includes an induction coil arranged adjacent to the induction lamp and a power source unit for supplying high-frequency electric power to the induction coil. High-frequency electromagnetic fields are generated within the bulb of the induction lamp by the induction coil. If arc discharge is generated within the bulb by the high-frequency electromagnetic fields, the discharge gas thus excited emits ultraviolet rays. A fluorescent material is coated on the inner surface of the bulb of the induction lamp. The ultraviolet rays are converted to visible rays by the fluorescent material, as a result of which the induction lamp emits light.
When starting up the induction lamp, it has been customary in the prior art that the frequency of the alternating current power outputted from a high-frequency power supply circuit to the induction coil is kept constant for a specified time period immediately after starting the supply of the alternating current power to the induction coil.
However, relatively high electrical stresses are apt to be developed in circuit parts, if the frequency of the alternating current power is maintained constant immediately after starting the supply of the alternating current power.
Further, an output voltage of a direct current power supply circuit is gradually increased up to a predetermined normal voltage after starting up the induction lamp. Then the output voltage is maintained at the normal voltage.
When starting up the induction lamp, it has been customary in the prior art that the supply of the alternating current power from the high-frequency power supply circuit to the induction coil is started after the output voltage of the direct current power supply circuit is stabilized at the normal voltage.
If the supply of the alternating current power is started after stabilization of the output voltage of the direct current power supply circuit at the normal voltage as set forth above, however, relatively high electrical stresses are developed in circuit parts at the moment of starting the supply of the alternating current power from the high-frequency power supply circuit to the induction coil.